Convert CONFIG_SYS_MALLOC_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MALLOC_LEN

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/configs/socfpga_n5x_vab_defconfig b/configs/socfpga_n5x_vab_defconfig
index cb3f3a9..a5241ee 100644
--- a/configs/socfpga_n5x_vab_defconfig
+++ b/configs/socfpga_n5x_vab_defconfig
@@ -6,6 +6,8 @@
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_ENV_SIZE=0x1000
 CONFIG_ENV_OFFSET=0x200
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x02000000
+CONFIG_SYS_MALLOC_LEN=0x500000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="socfpga_n5x_socdk"
 CONFIG_SPL_TEXT_BASE=0xFFE00000